1. Field of the Invention
This invention relates generally to an encapsulated flip chip package and a method for making the same. More particularly, this invention relates to a flip chip package having a thermally conductive member that maintains flatness and provides good heat dissipation.
2. Description of Related Art
The general construction of a plastic encapsulated semi-conductor device, also known as a plastic ball grid array (PBGA) package, is a semi-conductor chip mounted on an upper surface of a laminate substrate with a plurality of solder balls attached to a lower surface of the laminate substrate that can be bonded to a circuit card. Traditionally, the most common PBGA package has been a semi-conductor chip that is electrically connected to electrical circuitry on the laminate substrate by conventional wire bonds or loop wire bonds. The semi-conductor device has an over-molded plastic resin body that protects the semi-conductor chip and the wire bonds, and this over-molded dielectric may be up to about 20 mils thick above the surface of the chip in order to adequately protect the wire loops. Consequently, the wire-bonded PBGA has a relatively high thermal resistance, and the low heat dissipation makes the wire-bonded package unsuitable for high power applications.
Flip-chip PBGAs have been subsequently introduced and, if encapsulated, are made thinner than wire-bonded PBGAs for better heat dissipation. Flip chip packages generally consist of semi-conductor chips that have terminations in the form of solder pads or bump contacts that are disposed on the surface of the chip that is adjacent to the laminate substrate. Flip-chip packages derive their name from the apparent flipping of the chip to yield a chip orientation that is upside down compared to that of the wire-bonded PBGAs. Since the solder bumps are connected to the circuitry of the laminate substrate, there is no need for large wire loops or the thick application of dielectric that surrounds them.
A flip-chip package, however, typically requires an underfill material to keep moisture away from solder interconnections and to reinforce the solder joints which are prone to fatigue. The underfill material encompasses the solder interconnections between the chip and the laminate substrate. In addition to the underfill, the flip chip package may have a body that surrounds the chip. If the body surrounds only the periphery of the chip then it does not add significant thermal resistance to the package because the surface of the chip, opposite the surface that is electrically connected to the laminate substrate, is often left exposed. Such a structure, and flip chip packages without a body, are known as "bare-chip". While the bare-chip structure has enhanced thermal dissipation, dimensional stability is very difficult to control and warpage is a severe problem. The coefficient of thermal expansion (CTE) of the chip, for example a silicon chip, is about 3 ppm/.degree. C. whereas the CTE of a composite laminate substrate is about 20 ppm/.degree. C. During thermal cycling the chip restricts the expansion or contraction of the laminate substrate. The bending that is produced by thermal mismatch results in early fatigue failure of the ball grid array when the flip-chip package is attached to a circuit card or board. In severe cases the bending can cause fracture of the chip.
Flip-chip packages have also been made with a thermally-conductive covering, usually metal, to further improve heat transfer. The thermally-conductive covering is attached to the chip with a thermal coupler such as an adhesive, a thermal paste or a grease to improve the transfer of heat from the chip to the thermally conductive covering. A flip-chip package of this construction can have an additional problem since attachment of the thermally-conductive covering may result in delamination at the chip-to-covering interface. If the thermal coupler is substantially rigid the thermal coupler can fracture the chip, or if the thermal coupler is a non-rigid thermal grease, for example, it is prone to displacement during thermal cycling.
It is desirable to produce the flip-chip package that has low thermal resistance yet eliminates the need for a thermal coupler. It also desirable to produce a flip chip package that exhibits minimal warpage throughout the temperature range encountered in manufacture and in use to enhance ball grid array fatigue life.